Inspection of ski boots presently in the market shows that they comprise, in most cases, two distinct elements which are separately manufactured, namely, an inner slipper and an outer boot constituted by a shell and one or several portions of molded plastic material.
Various types of slippers for ski boots are known, among them (a) assembled slippers made either by sewing together of elements or by cutting up sheet or plate material, or by the injection of synthetic cellular resin between two walls of leather or other plastic material; and (b) slippers manufactured from plates of closed cell foam, attached together by sewing or soldering after having been heat-formed to the desired dimensions.
Also known is a slipper obtained by pressure molding of a cellular foam directly between the walls of a mold, as well as a manufacturing process which consists of injecting polyurethane foam between two walls placed in a mold and defining the general form of a slipper whose integral tongue is subsequently cut to form. Projections arranged on the mold limit at specific locations the regions into which the polyurethane foam is not to penetrate, and determine the locations of the least thickness suited, for example, for the cutting of a tongue.
However, all of these types of slipper have disadvantages which result in high costs of production, because of either the price of the materials used or of the labor required.
Thus, in the last-mentioned process, the walls which are to form the slipper consist of two socks, one inserted in the other, the outer sock having greater dimensions than the inner sock, so that a space is left between them. This arrangement is dictated by the fact that the materials used for this type of sock (PVC--coated fabric) are not very elastic. It is thus necessary to provide a free space between the socks so that the polyurethane mixture in liquid phase can flow between the walls and then expand to the point where it fills the entire interior space of the mold during the expansion thrust phase, without causing the seams and/or solder of the minimally elastic walls of the socks to burst.